1. Field of the Invention
This invention relates to a substrate for an electron source that can suitably be used for an image-forming apparatus such as a display apparatus.
2. Related Background Art
There have been known two types of electron-emitting devices; the thermionic electron source and the cold cathode electron source. Cold cathode electron sources refer to the field emission type (hereinafter referred to as the FE type), the metal/insulation layer/metal type (hereinafter referred to as the MIM type) and the surface conduction electron-emitting type. Examples of FE type device include those proposed by W. P. Dyke & W. W. Dolan, "Field emission", Advances in Electronics and Electron Physics, 8, 89 (1956) and C. A. Spindt, "Physical Properties of thin-film field emission cathodes with molybdenum cones", J. Appl. Phys., 47, 5248 (1976). Examples of MIM type devices are disclosed in papers including C. A. Mead, "Operation of Tunnel-Emission Devices", J. Appl. Phys., 32, 646 (1961). Examples of surface-conduction electron-emitting devices include the one proposed in M. I. Elinson, Radio Eng. Electron Phys., 10, 1290 (1965).
An surface-conduction electron-emitting device is realized by utilizing the phenomenon that electrons are emitted out of a thin film with a small area formed on a substrate when an electric current is forced to flow in parallel with the film surface. While Elinson proposes the use of SnO.sub.2 thin film for a device of this type, the use of Au thin film [G. Dittmer: "Thin Solid Films", 9, 317 (1972)], the use of In.sub.2 O.sub.3 /SnO.sub.2 thin film [M. Hartwell and C. G. Fonstad: "IEEE Trans. ED Conf.", 519 (1975)] and the use of carbon thin film [H. Araki et al.: "Vacuum", Vol. 26, No. 1, p. 22 (1983)] are also proposed. The applicant of the present patent application has proposed a number of improvements to surface-conduction electron-emitting devices.
FIGS. 19A and 19B of the accompanying drawings schematically illustrate a typical surface conduction electron-emitting device. FIG. 19A is a plan view and FIG. 19B is a cross sectional view. Referring to FIGS. 19A and 19B, it comprises a substrate 1, a pair of device electrodes 2 and 3 oppositely arranged on the substrate and a electroconductive film 4 connecting the device electrodes 2 and 3 and having an electron-emitting region 5 as part thereof. Conventionally, an electron-emitting region 5 is produced in a surface conduction electron-emitting device by subjecting the electroconductive film 4 of the device to an electrically energizing process, which is referred to as energization forming. In the energization forming process, a voltage is applied to the device electrode pair 2 and 3 that are connected by the electroconductive film 4 to partly destroy, deform or transform the film and produce an electron-emitting region 5 which is electrically highly resistive. Thus, the electron-emitting region 5 is part of the electron-emitting region-forming thin film 4 that typically contains a fissure therein so that electrons may be emitted from the fissure and its vicinity. While the voltage used for the energization forming may be a DC voltage or a voltage that gradually rises, a pulse voltage is preferably used to produce a device that operates satisfactorily for electron emission. A pulse voltage may have a constant wave height or a height that gradually rises depending on the circumstances under which the device is prepared.
In order to produce an electron-emitting region by means of energization forming that shows desired electron emitting characteristics, the electroconductive film is preferably made of electroconductive fine particles. While an electroconductive film may be formed from electroconductive fine particles in many different ways, in a known method, a solution containing an organic metal compound is applied to the surface of a substrate to produce a film of the organic metal compound there, which is then subjected to a heat treatment to produce an electroconductive film comprising fine particles of metal and/or metal compound. This technique is advantageous particularly in terms of manufacturing cost because, unlike a gas deposition method, it does not require the use of a large vacuum system. There is also known a method with which the solution is applied only to areas for forming a film by means of an ink-jet system. This method is advantageous in that it does not require an independent step of patterning the electroconductive film.
The electroconductive film of a surface-conduction electron-emitting device may be made of PdO fine particles prepared by heat treating an organic compound of palladium in the atmosphere. The heat treatment is typically conducted at 300 to 400.degree. C. for a little more than 10 minutes. PdO is a substance that shows an appropriate electroconductivity and hence is suited for an electron-emitting region that is produced by energization forming. Additionally, PdO can easily be reduced to produce Pd when it is heated in a vacuum or exposed to reducing gas. After producing an electron-emitting region in an electroconductive film by energization forming, the electric resistance of the electroconductive film can be lowered by two digits by reducing the PdO to Pd. There are cases where a surface-conduction electron-emitting device having an electron-emitting region operates better when the electroconductive film shows a low electric resistance. The PdO will be reduced advantageously in such a case.
The applicant of the present patent application has also proposed various electron sources prepared by arranging a number of electron-emitting devices of the above described type on a substrate and image-forming apparatus comprising such an electron source. In order to arrange an electron source in a vacuum envelope, the electron source and the envelope have to be firmly bonded with other components. The bonding operation is generally carried out by means of frit glass and by heating them to about 400 to 500.degree. C. for a period between 10 minutes and an hour depending on the size of the envelope and other factors so that the frit glass may be molten to firmly hold them together.
The envelope is preferably made of glass containing sodium such as soda lime glass that can be easily bonded with frit glass. The use of soda lime glass is particularly advantageous because it is not expensive. It is also preferable that the substrate is made of soda lime glass because it has to show a thermal expansion coefficient close to that of the envelope and should be bonded reliably to the envelope.
However, a substrate made of soda lime glass can be accompanied by the following problems.
Firstly, soda lime glass contains alkali metals, particularly sodium (Na) in the form of Na.sub.2 O, to a large extent. Since sodium can be easily diffused by heat, the sodium contained in the substrate of soda lime glass is diffused into various components of the image-forming apparatus during the processes where the substrate is exposed to high temperature to adversely affect the components.
Additionally, the substrate of the electron source is exposed to higher temperature when it is bonded to the envelope by means of frit glass so that the sodium diffusion becomes even more remarkable to aggravate the adverse effect.
However, the use of a material other than soda lime glass, quartz for example, has to be avoided because such a material is costly and provides additional problems including the difficulty of bonding.
A technique that has been proposed to bypass the above problems is to form an SiN (Japanese Patent Application Laid-Open No. 8-162001) or SiO.sub.2 film on the surface of the soda lime glass substrate by means of sputtering. However, stress can be generated between the soda lime glass substrate and the SiN or SiO.sub.2 film that can eventually separate them particularly if the SiN or SiO.sub.2 film is thick. On the other hand, a thin SiN or SiO.sub.2 film cannot satisfactorily suppress the sodium diffusion when it is exposed to high temperature for a long time during the process of manufacturing an electron source.
Additionally, a sputtering operation for forming an SiN or SiO.sub.2 film requires the use of a large sputtering system relative to the size of the substrate and consequently raises the manufacturing cost.
Therefore, there is a strong demand for a technology of effectively avoiding the adverse effect of sodium diffusion.
A similar problem is caused by diffusion of sulfur from the substrate and measures for preventing sulfur diffusion is also sought.